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Parasites comprise a substantial proportion of global biodiversity and exert important ecological influences on hosts, communities and ecosystems, but our knowledge of how parasite populations respond to human impacts is in its infancy. Here, we present the results of a natural experiment in which we used a system of highly successful marine protected areas and matched open‐access areas in central Chile to assess the influence of fishing‐driven biodiversity loss on parasites of exploited fish and invertebrate hosts. We measured the burden of gill parasites for two reef fishes (Cheilodactylus variegatus and Aplodactylus punctatus), trematode parasites for a keyhole limpet (Fissurella latimarginata), and pinnotherid pea crab parasites for a sea urchin (Loxechinus albus). We also measured host density for all four hosts. We found that nearly all parasite species exhibited substantially greater density (# parasites m⁻²) in protected than in open‐access areas, but only one parasite species (a gill monogenean of C. variegatus) was more abundant within hosts collected from protected relative to open‐access areas. These data indicate that fishing can drive declines in parasite abundance at the parasite population level by reducing the availability of habitat and resources for parasites, but less commonly affects the abundance of parasites at the infrapopulation level (within individual hosts). Considering the substantial ecological role that many parasites play in marine communities, fishing and other human impacts could exert cryptic but important effects on marine community structure and ecosystem functioning via reductions in parasite abundance.

Pea crabs are globally ubiquitous symbionts in the marine environment that cause serious economic impact in the aquaculture production of several major bivalve species. However, little is known about their host-parasite interactions, especially the mating behaviour of these parasites that could prove useful for controlling their infestation in aquaculture. In this study, the mate location behaviour of male New Zealand pea crabs, Nepinnotheres novaezelandiae (Filhol, 1885), was observed when dwelling in its preferred host, the commercially important green-lipped mussel, Perna canaliculus. Given the cryptic behaviour of the male crabs, a novel trapping system was developed to determine whether male crabs would exit their mussel hosts in response to an upstream female crab. The presence of receptive female crabs placed upstream successfully attracted 60% of male crabs from their host over 24 h. Observations of the nocturnal mate-finding behaviour of male crabs were made in darkness using infrared video recordings. Males spent on average 49 min on empty hosts and never left a mussel containing a female conspecific once found, spending 200 min on average to gain entry to the mussel. Male crabs were often observed stroking the mantle edge of the mussel whilst attempting to gain entry, successfully increasing mussel valve gape during entry from 3.7 to 5.5 mm. A pheromone-based mate location system is likely used by this crab to greatly reduce the risks associated with the location of females. Les crabes petit pois sont globalement des symbiotes omniprésents dans le milieu marin, qui ont de graves conséquences économiques dans la production par l’aquaculture de plusieurs espèces majeures de bivalves. Cependant, on en sait peu sur leurs interactions hôte-parasite, en particulier le comportement d’accouplement de ces parasites, qui pourraient s’avérer utile pour contrôler leur infestation dans l’aquaculture. Dans cette étude, le comportement de recherche de partenaire sexuel par le crabe mâle, Nepinnotheres novaezelandiae (Filhol, 1885), a été observé chez leur hôte préféré, la moule verte d’importance commerciale Perna canaliculus. Étant donné le comportement cryptique du crabe mâle, un nouveau système de piégeage a été mis au point pour déterminer si les crabes mâles quittent leurs hôtes moules en réponse à un crabe femelle en amont. La présence de crabes femelles réceptives situées en amont a réussi à attirer 60 % des crabes mâles de leur hôte sur 24 heures. Les observations du comportement nocturne de recherche de partenaire sexuel des crabes mâles ont été faites dans l’obscurité en utilisant des enregistrements vidéo infrarouge. Les mâles ont passé en moyenne 49 min sur des hôtes vides et n’ont jamais quitté une moule contenant un congénère femelle une fois trouvée, passant 200 min en moyenne pour entrer dans la moule. Les crabes mâles ont été souvent observés en train de caresser le bord du manteau de la moule pour essayer d’entrer, augmentant avec succès l’entrebâillement de la moule pendant leur entrée de 3.7 à 5.5 mm. Il est probable qu’un système de localisation de partenaire sexuel à base de phéromones est utilisé par ce crabe pour réduire considérablement les risques associés à la recherche des femelles.

Species of the family Pinnotheridae constitute an ideal group for morphometric studies due to their complex morphological adaptations. These adaptations respond to the selective pressure of a symbiotic life style. This study describes the relative growth and morphometric features of the symbiotic pea crab Austinixa aidae (associated with the ghost shrimp Callichirus major), from the sandy beaches in the southwest Atlantic, Brazil. Significant differences were detected in the biometric proportions, particularly the chelar propodus length and carapace width, of each sex. These dimensions were also related to the size at which the individuals reached morphological sexual maturity (5.1 mm of carapace width for both sexes). Males and females were 2.4 times wider than long, which corresponds to the principal adaptation developed by Austinixa species to live in cryptic environments. Moreover, juveniles were proportionally more rounded. The changes in the biometric proportions of carapace length and width of A. aidae were more pronounced in males and females, adaptations that facilitate roaming within the galleries of their hosts.

The ecology of the pea crab Pinnixa tumida living in the endobenthic holothurian Paracaudina chilensis, which buries its body in the fine sandy bottom in the sublittoral zone, was investigated. Almost all of the pea crabs collected from their host holothurians were mature females. Mature pea crabs were found to live singly in the alimentary canal of the larger hosts, which are mainly distributed just below the low water level of the spring tide. However, immature crabs were not found in either large or small hosts. In the laboratory, mature crabs searched for the tip of the host's tail, which appeared on the sand's surface, and then entered the host's alimentary canal through the opening of the anus at the tail's tip. These results indicate that the pea crab enters larger hosts only when it is mature, or just before maturity. Pea crabs showed 2 types of feeding preference in the laboratory: suspended particles and the mucus secreted in the alimentary canal of the host. Ovigerous females were found in the hosts between late February and mid-May, carrying eggs which developed slowly and synchronously with those of almost all of the other ovigerous females observed throughout this period, indicating that the pea crab has an annual breeding cycle. The biology of the pea crab was compared with that of crabs living in bivalves and on irregular echinoids.